It is a well-known fact that the passage of a conducting material, for instance a coin, through an electromagnetic field results in changes in the oscillating circuit associated the such field's exciters, affecting oscillating amplitude and frequency.
Obviously, interaction of the conducting part with the electromagnetic field shall depend upon its dimensional characteristics, for instance, diameter, thickness and snarling, as well as the electrical characteristics thereof, viz. electrical conductivity and magnetic permeability of the alloy.
It is possible to identify the coin being examined by analyzing oscillator variations during the time taken by the coin to go past the sensors.
Different systems of analyzing oscillator variations when the conducting piece goes past the sensors or exciters are known, which establish the maximum variation of the amplitude signal or the maximum drift of the frequency signal, or else and calculate averages of different readings over a given period of time. At all events, signal analysis is not complete and the information obtained is not enough to differentiate between given types of coins or cards.